1. Field of the Invention
The present invention generally relates to optical scanning devices and image forming apparatuses.
2. Description of the Related Art
Optical scanning devices forming a beam spot on a surface to be scanned (hereinafter referred to as a “scan surface”) and optically scanning the scan surface with the beam spot, and image forming apparatuses using such an optical scanning device are widely known in association with image forming apparatuses such as an optical printer (laser printer), optical plotter (laser plotter), digital copying machine and facsimile machine. In such optical scanning devices, the beam spot is formed through deflecting a light beam from the light source side by an “optical deflecting part” such as a rotary polygon mirror, and focusing the deflected light beam toward the scan surface by a “scanning and imaging optical system” such as an fθ lens.
Recently, not only optical scanning devices that optically scan one scan surface, but also optical scanning devices that simultaneously and optically scan a plurality of scan surfaces are in practical use. In other words, in addition to an optical scanning device using a conventional single beam scanning method, an optical scanning device using a multi-beam scanning method is being realized as the optical scanning device used for the image forming apparatus. As for the image forming apparatus, besides an image forming apparatus forming a monochrome image, an image forming apparatus forming a color image and a multicolor image is coming into practical use. Especially, a “tandem-type” image forming apparatus is actively being developed (refer to Japanese Laid-Open Patent Applications No. 11-157128, No. 9-127443, No. 9-54263, No. 2001-4948, No. 2001-10107 and No. 2001-33720).
In addition, “sharing in common a part of an optical system that forms light paths from light sources to respective (different) scan surfaces” for optically scanning a plurality of scan surfaces is performed (for example, Japanese Laid-Open Patent Applications No. 2001-4948, No. 2001-10107, No. 2001-33720 and No. 2001-343603).
Higher density writing by optical scanning devices is being developed, and realization of writing densities of 1200 dpi, 1600 dpi and higher is intended. In order to achieve high-density writing, the stability of a beam spot, that is, “to prevent the spot diameter of a beam spot, which optically scans a scan surface, from being greatly varied according to image height” is essential.
One of the causes for the variation of the spot diameter of the beam spot according to image height is, as is generally known, “field curvature caused by the scanning and imaging optical system”. There are a large number of known “scanning and imaging optical systems substantially correcting the field curvature” so as to increase the stability of the beam spot.
In an optical scanning device using the multi-beam scanning method, in addition to the stability of the beam spot, it is also important that imaging magnification of the scanning and imaging optical system, which focuses deflected light beams on a scan surface, be substantially constant.